This invention relates to an apparatus and method for cathodically protecting a metal structure against corrosion using a cathodic protection system. In another aspect this invention relates to a method and apparatus for directly indicating the functional condition of a cathodic protection system. In still another aspect this invention relates to indicators which operatively function with a cathodic protection system so as to denote the exact point at which the system needs replacing.
Cathodic protection systems are employed to prevent corrosion of metal structures exposed to an electrolytic environment. Cathodic protection can be effected for marine or subterranean corrodible structures by electrically connecting the corrodible structure to sacrificial anodes constructed of a metal that is higher in the electromotive series than the protected structure, i.e. a metal that is anodic to the material of the protected structure. When the protected structure and the electrically connected sacrificial anode are both disposed within the same electrolytic environment (e.g., earth or water containing free positive ions), a galvanic cell is formed in which the protected structure is the cathode.
Metal atoms on the exposed surface of the sacrificial anode are ionized by the surrounding electrolyte and go into solution with the electrolyte, thereby corroding the sacrificial anode. Due to the difference in electrical potential between the cathodically protected metal and the sacrificial anode, electrons produced by the electrochemical corrosion reaction of the anode flow as an electrical current through the electrical connection between the sacrificial anode and the protected structure. When electrons reach the protected structure, they combine with positive ions in the electrolyte at the surface of the protected structure. The protected structure does not corrode since the positive ions associate with the free electrons readily available at the surface of the protected structure, which positive ions would otherwise initiate a corrosion reaction at the surface of the protected structure.
Often, the function of a cathodic protection system is supplemented by applying a protective coating to the exterior of the cathodically protected structures to reduce the exposure of the protected structure to the electrolytic environment. However, a protective coating to the exterior of the cathodically protected structures will not completely isolate the protected structure from the electrolyte since small cracks or discontinuities in the coating develop as the coating ages, allowing the portion of the structure exposed through the cracks to be corroded. Further, such a coating is incapable of perfectly isolating the corrodible structure from positive ions in the surrounding electrolyte as some of the positive ions are capable of diffusion or migration through the protective coating itself.
Cathodic protection systems are capable of protecting the corrodible structure from corrosion so long as a sufficient amount of sacrificial anode metal remains to supply electrons to the protected structure. When an anode is nearly completely corroded, it must be replaced in order for the cathodic protection system to continue its function. The corrosion rate of the sacrificial anode, and thus the point in time when the anode needs to be replaced, is difficult to predict since it is influenced by a number of variable factors such as for example the composition of the surrounding soil or water and localized variations in that composition.
Although the prior art has devised a number of schemes for determining the condition of a cathodic protection system to ascertain whether the system is functioning and/or for determining when the sacrificial anodes are in need of replacement, these schemes cannot be effectively interchanged to operate in both a subterranean as well as a marine environment. For example, in order to cathodically protect a considerable length of subterranean pipe it is necessary to have a plurality of anodes electrically connected to the pipe and spaced along the length of the pipe. The condition of a subterranean cathodic protection system is conventionally monitored by determining the polarity and/or magnitude of the electrical potential of the sacrificial anode and/or the electrical potential of the protected pipe with respect to a reference half-cell disposed in the electrolyte surrounding the pipe. These determinations must be made at a plurality of locations along the length of the pipe to determine the condition of the entire system. To facilitate these monitoring test, electrical connections, in the form of an insulated electrical conductor electrically connected to the sacrificial anode and/or the cathodically protected structure and routed to the surface of the electrolyte in which the system is disposed, are provided at various points along the pipe, e.g. at each connection between a sacrificial anode and the cathodically protected pipe. Because of the expense and physical impracticality of stabilizing such monitoring schemes on water surfaces, such prior art monitoring systems, such as the one described above, may be satisfactory for subterranean cathodic systems however, the conventional way to monitor marine systems continues to be the physical inspection thereof by underwater divers.
In spite of its acceptance and wide use, underwater inspection has inherent shortcomings which influence and affect the data obtained. Visability of the driver appears to be the most critical of a number of factors which influence the validity of the data obtained in underwater inspection. For example, in shallow waters and near the ocean floor, which is where most cathodic protection systems are located, sediments are often suspended in a layer of murky water, a diver's vision may be significantly reduced, even with the aid of additional lighting. Because the visual conditions at the inspection site are oftentimes hampered by unstable water conditions, the use by the diver of conventional equipment requiring visual interpretation, an example of such is a portential meter, is most unreliable since near ideal conditions must be present for an effective use thereof. These inescapable water conditions pose a continuous problem inasmuch as adequate cathodic protection of offshore pipe lines is dependent on obtaining reliable data on which to base decisions for replacing dysfunctional systems.
It is therefore an object of this invention to provide an apparatus and a method for cathodically protecting a metal structure against corrosion which is submerged in an electrolyte. Another object of this invention is to provide a method and apparatus for indicating when a cathodic protection system is dysfunctional. Still another object of this invention is to provide a method and apparatus for indicating the exact point at which a cathodic protection system needs replacing.